Unraveling the Multielectron Redox Reactions of β-VOPO₄ for Sodium Ion Batteries

Realizing multielectron redox reactions is a promising strategy to improve the specific capacity and energy density of rechargeable batteries. Although reversible two-electron redox has been demonstrated for a few cathode materials for Li-ion batteries, few examples are known for Na-ion batteries. Here, we have investigated the two-electron redox of β-VOPO₄ for Na-ion intercalation via a combination of X-ray diffraction and absorption spectroscopic methods to elucidate the evolution of crystal and electronic structures. The insertion of two Na ions per formula unit of β-VOPO₄ results in the irreversible structural phase transition to a new, expanded orthorhombic Na₂VOPO₄ phase, which can be reversibly cycled to an orthorhombic NaVOPO₄. However, further extraction of Na ions from this new orthorhombic NaVOPO₄ phase could not be realized even after charging to 4.3